1. Field of the Invention
This invention relates generally to honeycomb panels, and relates more particularly to a honeycomb panel that is readily formable by virtue of a porous, rather than solid, skin panel.
2. Description of the Relevant Art
A honeycomb panel consists of a core material sandwiched between and bonded to two skin panels. The core material is light in weight and, typically, is composed of a cellular material having cells that extend transversely between the two skin panels. The skin panels are typically thin, solid sheets of material, commonly metal, that are adhesively bonded to both sides of the core layer.
Honeycomb panels are light in weight but very stiff in response to bending loads. The core layer separates the two solid skin panels by a distance equal to the thickness of the core layer, thereby providing a relatively large moment of inertia about a principal axis parallel to the plane of the skin panels. The skin panels take the bulk of the tensile and compressive loads generated in bending.
Since a honeycomb panel is a multi-piece assembly of the core layer and the two skin panels, the fabrication cost of honeycomb panels can be significant. For that reason, the utilization of honeycomb panels has been limited to those applications for which the advantages of lightness and stiffness outweigh the disadvantage of high fabrication cost.
Honeycomb panels are used extensively in the aircraft and aerospace industries, but such panels often must be curved or otherwise configured in shapes other than flat. Honeycomb panels are typically fabricated by forming the core layer to a desired contour, then coating the core layer or skin panels with adhesive, positioning the two skin panels on either side of the core layer, and then placing this assembly in a curing press, which laterally compresses the panel until the adhesive is cured. In fabricating curved honeycomb panels, the curing press utilizes a specially shaped die that holds the pre-bond assembly at the desired shape while the adhesive cures. The skin panels may be preformed to their desired contours prior to loading into the curing press.
If the honeycomb panel is flat rather than curved, significant reductions in fabrication cost can be realized by using a flat press and curing several panels simultaneously. The lower cost of tooling and increased production throughput of this technique significantly decreases the production cost of flat honeycomb panels as compared to curved panels. The disadvantage to flat curing is that conventional flat honeycomb panels can not be readily formed into curved panels due to the inherent stiffness of the panels. Attempts to form curved honeycomb panels from flat panels typically result in destroying the adhesive bonds or collapsing the core layer, thereby ruining the honeycomb panel.
Conventional honeycomb panels have not been used extensively in the automobile industry due to cost and finish considerations. In ground-based transportation vehicles, the weight savings of conventional honeycomb panels in comparison to conventional solid metal panels is not significant enough to offset the higher cost of fabrication. Honeycomb panels in such applications often need to be curved, thereby precluding the use of the above-described, low-cost method for fabricating flat honeycomb panels. In addition, the outer surface of a conventional honeycomb panel does not provide a smooth enough surface for use as an exposed panel, given the high quality of surface finish that the automotive industry demands.
In certain instances, the automobile industry has switched from stamped sheet metal body panels to molded plastic body panels. Such plastic body panels typically are constructed in two pieces, an outer cosmetic panel and an inner reinforcing panel, both molded from sheet molding compound (SMC), which is usually composed of a resin reinforced with fibrous glass material. The cosmetic panel usually has a uniform thickness because abrupt changes in panel thickness would be visible in the outer surface. Since the cosmetic panel is not stiff enough to use by itself, the reinforcing panel is bonded to its back side, thus providing a moderately stiff assembly of perhaps 5/8 to 3/4 of an inch in total thickness. The reinforcing panel typically includes ribs or other structural features designed to increase the stiffness of the assembly without unduly increasing its weight.